SAS watchout

Why do I have to correct EPIC-pn Timing Mode exposures for X-ray Loading (only necessary for observations performed until 23 May 2012)?

X-ray Loading (XRL) occurs whenever source counts contaminate the offset map taken prior to each EPIC-pn exposure. Instrument operations have been configured such that XRL should never occur, if the appropriate combination of instrument mode and filter is selected based on the brightness of the primary target. However, it has been discovered that almost all exposures taken in EPIC-pn Fast Modes (Burst and Timing Mode) until the 23 May 2012 were unexpectedly affected by XRL, see Fig. 1 in XMM-SOC-CAL-TN-0083

As of that date offset maps in these modes are being taken with the optical blocking filter in the CLOSED position, thus preventing source contamination.

For EPIC-pn Timing Mode observations taken until 23 May 2012 it is therefore recommended to apply the XRL correction. This is done by running the SAS meta-tasks epproc or epchain with the following input parameters:

epchain runepreject=yes withxrlcorrection=yes datamode=TIMING

Note that these parameters are not the default settings in SAS 13.5, or earlier SAS versions.

These input parameter values ensure that epreject applies the XRL correction to the intermediate event lists produced by the reduction meta-task. We discourage users from using interactively epreject for this purpose.

Also pipeline generated products of those EPIC-pn Timing Mode observations, available in the XMM-Newton Science Archive (XSA), should be re-processed in the same way.

No XRL correction is available for EPIC-pn Burst Mode exposures yet. Trying the aforementioned command on an exposure taken in a mode different from Timing Mode yields an error.